This podcast talks about functional magnetic resonance imaging (FMRI)allowing scientist to watch areas inside the brain.It show them where the brain certain responses reside like fear reside. The FMRI has limitation. The certain brain regions leads to increase blood flow to active areas that is what you see in the FMRI. Scientist only knew that a correlation not causation meaning they were not sure if the blood would increase. So they made an experiment on rats to see what would happen they were able to turn on genetically engineered brain cells to the rats using a blue light delivered directly flowed indeed match precisely those excited cells.